Allow users to set profile picture

Currently the bot will place the default logo as profile picture if it can't access the users profile picture. Apart from no profile picture being set, this can also be the case when the users privacy settings don't allow the bot to access it and users having to change their privacy setting just to have the bot access the picture doesn't sound right.

So one could

• add a ConversationHandler for setting a custom fallback profile picture. Fallback here means that the user will take the actual profile picture, if available - that's more the spirit of this bot IMO
• Commandhandlers to show and unset the fallback profile picture
• Refactor how user info is stored. Currently, I'm just storing the User object and adding some custom object to it. As PTB recently added __slots__, the latter won't work anymore after the next ptb release (or at least raise warnings). Moreover with ptb#2262 we'll have a neat way to store the required info for the user. So basically, wait for the next ptb release before going for this.